1. Field of the Invention
This invention relates to a thick film thermal head for making a heat-sensitive stencil master.
2. Description of the Related Art
Thermal heads for making a heat-sensitive stencil master generally comprise an array of resistance heater elements and imagewise perforate a stencil master material by selectively energizing the heater elements and are broadly classified into thin film thermal heads and thick film thermal heads by the structure.
FIG. 9 shows an example of a conventional thick film thermal head. In FIG. 9, the conventional thick film thermal head comprises a ceramic substrate 95, a heat insulating layer 92 formed on the ceramic substrate 95, a plurality of comb-tooth electrodes 94 which are formed on the heat insulating layer 92 and arranged in a row in a direction perpendicular to their longitudinal directions, and a resistance heater 91 which extends in the direction of the row of the comb-tooth electrodes 94. The resistance heater 91 is formed over the comb-tooth electrodes 94 to continuously extend across the electrodes 94. With this arrangement, when an electric voltage is applied across a pair of adjacent electrodes, the part of the resistance heater 91 between the electrodes 94 generates heat. That is, parts 91a of the resistance heater 91 between adjacent electrodes 94 form resistance heater elements. The resistance heater elements 91a are arranged in the direction of the row of the comb-tooth electrodes 94. This direction will be referred to as "the main scanning direction", hereinbelow.
When making a stencil master by a thermal head, it is generally necessary to form, on a stencil master material 71, perforations 72 which are separated from each other in both the main scanning direction X and the sub-scanning direction Y (the direction substantially perpendicular to the main scanning direction) as shown in FIG. 7.
If the perforations 72 are continuous in the main scanning direction X as shown in FIG. 8, ink is supplied to the printing paper through the perforations in an excessive amount, which results in offset and/or strike through.
If each of the resistance heater elements 91a is thermally insulated from the parts of the resistance heater 91 on opposite sides of the resistance heater element 91a in FIG. 9, the width W in the main scanning direction of the area which generates heat when the element 91a is energized will be substantially equal to the space P between adjacent electrodes 94.
However, actually each of the resistance heater elements 91a is not thermally insulated from the parts of the resistance heater 91 on opposite sides of the resistance heater element 91a, and accordingly, heat generated from each resistance heater element 91a propagates to the part adjacent thereto, whereby, as shown in FIG. 10, an area 101 wider than the resistance heater element 91a, or the space P between adjacent electrodes 94, becomes hot. When adjacent two resistance heater elements 91a are simultaneously energized, the actual heat generating areas 101 of the adjacent resistance heater elements 91a become closer to each other as shown in FIG. 10, which can result in continuous perforations.
Especially in the case of a thick film thermal head, since the resistance heater 91 is large in thickness, heat generated from each resistance heater element 91a spreads over a wide area while it propagates to the surface of the element 91a and the width W of the heat generating area more tends to become larger than the space P between the electrodes 94 at the surface of the resistance heater 91. Accordingly, in the case of a thick film thermal head, the aforesaid problem is more serious.